Dan,
I'm going to give a +1 to this package, but first I need you clarify 2 things for me:
1) BuildRequires: mate-file-manager-devel mate-file-manager-extensions, since the -extensions and -devel originate from the same sources why isn't -devel pulling the extensions (ex: through a Requires)? This would be in my opinion a good practice.
2) The style on the spec in different from previous packages; This isn't for sure an issue, so there's no big deal with this.
If you believe 1) can be improved, it would be nice; otherwise, if no one else objects, I let it slip in.

Dan,
I have reworked the spec file to fix most issues I've found, and I'm happy I haven't pushed this forward as some issues passed.
http://nmarques.fedorapeople.org/882559/mate-file-manager-image-converter.spec
You have there the spec re-worked, see if you can use it or rework the current spec to address the following issues:
1) %post/%postun entries are not required; we don't install any files in the path of the dynamic linker;
2) Improved summary and description;
3) Removed find entry for static blobs; --disable-static is working properly, no static blobs are being generated;
4) Add mate-file-manager for runtime dependency; this is disputable, but the dependencies added by RPM can be complemented.
About 2) I don't really care, but the others are need to be fixed, so you can either use 'ipsis verbis' my spec or you can rework yours to meet the criteria.
Let me know so I can run stuff again. And please no more 'We can fix that later', later is now :)